This invention relates generally to metal matrix composites and a process for making such composites. More specifically, this invention relates to copper/refractory metal matrix composites for use in microelectronic packages.
A metal matrix composite is a material that contains at least two metals, one of which is copper and the other is a refractory metal having a substantially higher melting point. Copper/refractory metal matrix composites are commonly used as substrates or carriers within microelectronic packages. A microelectronic package generally includes a heat-generating component, such as, for example, a chip die that is mounted onto a metal matrix composite used as a substrate. The metal matrix composite functions to transfer heat away from the microelectronic package while also maintaining dimensional stability under wide temperature fluctuations. The thermal conductivity and the coefficient of thermal expansion (CTE) of the composite can be controlled by varying the copper/refractory metal weight ratios based upon the design considerations of various end use applications.
Copper/refractory metal composites can be economically produced using powder metallurgy (xe2x80x9cP/Mxe2x80x9d) technology. Metal matrix composites are typically manufactured using either the xe2x80x9cinfiltrationxe2x80x9d or the xe2x80x9cpress and sinterxe2x80x9d process, and there are different advantages associated with each. In the infiltration process, refractory metal is press-molded into a shape and sintered to form a highly porous article. Molten copper is then infiltrated into the voids and interstices of the refractory metal to form a dense copper/refractory metal matrix composite. The infiltration process generally cannot be used to produce net shape parts. Rather, the parts produced by infiltration must be either pressed again via a second compaction process or be machined into final shape. In the press and sinter process, powdered copper and refractory metals are blended in prescribed proportions and then compacted and sintered to provide net shape or near net shape metal parts. Such parts can achieve composite densities greater than 99% of theoretical density, with a concomitant porosity of less than about 1%. One press and sinter process is described in U.S. Pat. Nos. 5,686,676 and 5,993,731 to Jech et al. These patents teach that when a compact of metal particles contain oxygenated copper, improved particle rearrangement and densification is achieved during sintering.
A disadvantage of the press and sinter process, however, is that the powder mix typically requires the addition of sintering aids, such as, for example, cobalt, iron or nickel. Sintering aids have many processing advantages, but they are known to deleteriously affect the thermal conductivity of the finished composite, even in small concentrations. Sintering aids are used to reduce the porosity and increase the density of the sintered parts by improving the diffusion of the copper and refractory metal powders during the sintering process. Sintering aids also improve processing conditions by achieving good metal diffusion throughout a wider sintering temperature range. The wider processing window allows for improved consistency in the quality of the production parts. Copper/refractory metal composites made without a sintering aid generally have a higher thermal conductivity than a composite containing the same copper concentration with sintering aid. However, composites that are made without a sintering aid are unacceptable in many applications because they can have a porosity of up to 5%. In microelectronic packaging applications, for example, a measured porosity of less than about 1% is generally required. Voids are particularly noticeable if the composite is machined or cut. Even if the composite is not cut, problems associated with porosity are manifested in defects such as blisters and staining during plating. A porosity of about 2% or more negatively affects the physical characteristics of copper/refractory metal composites, including for example, thermal conductivity. (See, xe2x80x9cDensification, Microstructural Evolution, and Thermal Properties of Liquid Phase Sintered Compositesxe2x80x9d, Johnson, J. L., PhD Thesis in Engineering Science and Mechanics, The Pennsylvania State University, Department of Engineering Science and Mechanics, August 1994.) Thus, although composites made with sintering aids have a lower thermal conductivity they are, nevertheless, preferred in many applications because of their comparatively low porosity.
U.S. Pat. Nos. 5,889,220 and 5,905,938 disclose that copper/tungsten (xe2x80x9cCu/Wxe2x80x9d) composites containing small quantities of phosphorus, i.e., 0.07% by weight or less, and cobalt as a sintering aid have a thermal conductivity that is useful in microelectronic applications. Specifically, U.S. Pat. No. 5,889,220 shows that the thermal conductivity of sintered Cu/W composites containing 10% copper and 0.2% cobalt is improved when the amount of phosphorus is less than 0.05% by weight, but is detrimentally affected by an amount of phosphorus of 0.05% or greater. The thermal conductivity of sintered Cu/W composites containing 20% copper and 0.2% cobalt is improved when the amount of phosphorus is 0.07% by weight or less, but is detrimentally affected by greater amounts of phosphorus. These patents show that a problem exists in achieving a high thermal conductivity when 10% and 20% copper/tungsten composites are made with 0.2% cobalt and contain an amount of phosphorus that is greater than 0.07% by weight.
It is desirable, therefore, to provide metal matrix composites and a process for making such composites, that have low porosity, high density and an improved thermal conductivity. It is desirable to provide copper/refractory metal composites that have improved thermal conductivity and low porosity when the sintering aid is about 0.2% by weight or greater. It is also desirable to provide microelectronic packages that achieve better dissipation of heat. It is further desirable to provide a process for making high quality net shape or near net shape metal matrix composites having improved thermal conductivity and a porosity of less than about 1% in a consistent manner.
The present invention provides copper/refractory metal matrix composites, and a process for making such composites, that have improved thermal conductivity and a maximum porosity of about 1%. The press and sintered copper/refractory metal composites of this invention containing sintering aid and phosphorus additives have improved thermal conductivity compared to similar press and sintered copper/refractory composites containing the same amount of sintering aid but no phosphorus. Also, the thermal conductivity of the composites herein approaches, or is nearly equivalent to, the thermal conductivity of copper/refractory metal composites that contain no additives, i.e., composites that contain the same amount of copper, but the balance is refractory metal. The performance, in terms of thermal conductivity, depends upon the weight ratio of phosphorus and sintering aid (the xe2x80x9cphosphorus/sintering aid ratioxe2x80x9d) in the composite. Surprisingly, it has been found that it is the weight ratio of phosphorus and sintering aid present in the composite that affects the thermal conductivity of the composite rather than the individual amounts of phosphorus and sintering aid.
The thermal conductivity of a composite of the present invention is improved when the phosphorus/sintering aid ratio is from about 0.25 to about 0.55, preferably from about 0.27 to about 0.55, more preferably from about 0.28 to about 0.45, even more preferably from about 0.27 to about 0.4, and yet even more preferably from about 0.3 to about 0.38. The copper/refractory metal composites of the present invention comprise by weight from about 5% to about 30%, preferably from about 8% to about 22%, more preferably from about 9% to about 21%, even more preferably from about 10% to about 20% and yet even more preferably from about 10% to about 15% copper; from about 0.2% to about 0.6%, preferably from greater than about 0.2% to about 0.5%, more preferably from about 0.25% to about 0.45%, even more preferably from about 0.28% to about 0.4%, and even yet more preferably from about 0.3% to about 0.4% sintering aid; from about 0.08% to about 0.3%, preferably from about 0.09% to about 0.28%, more preferably from about 0.9% to about 0.24%, and even more preferably from about 0.09% to about 0.16% phosphorus; and the remainder is refractory metal.
In one embodiment of the invention the copper/refractory metal composites contain cobalt as the sintering aid and tungsten as the refractory metal. The copper/tungsten (Cu/W) composites having a copper concentration of about 5%, 10%, 15%, 20% and 30% copper have a thermal conductivity of up to about 153 W/mK, about 180 W/mK, 204 W/mK, about 212 W/mK and about 224 W/mK, respectively.
In another embodiment of the present invention the copper/refractory metal composites are used as substrates to which heat-generating components can be attached, such as, for example, a chip die in the production of microelectronic packages. The substrates made from the copper/refractory metal composites of the present invention have improved thermal conductivity and low porosity, thereby improving the dissipation of heat generated within the package.
The process for making copper/refractory metal composites of the present invention comprises: selecting powdered metals comprising copper, sintering aid, phosphorus and refractory metal, at least one of the copper and the refractory metal containing chemically-bound oxygen in an amount sufficient to improve a sintering process; mixing the powdered metals to produce a mixture of powdered metals, forming a green compact with the mixture of powdered metals, and sintering the green compact containing chemically-bound oxygen in a reducing atmosphere to form a copper/refractory metal composite comprising by weight from about 5% to about 30% preferably from about 8% to about 22%, more preferably from about 9% to about 21%, even more preferably from about 10% to about 20%, and yet, even more preferably from about 10% to about 15% copper, from about 0.2% to about 0.6%, preferably from greater than about 0.2% to about 0.5%, more preferably from about 0.25% to about 0.45%, even more preferably from about 0.28% to about 0.04%, and yet even more preferably from about 0.3% to about 0.4% sintering aid, from about 0.08% to about 0.3%, preferably from about 0.09% to about 0.28%, more preferably from about 0.09% to about 0.24%, and even more preferably from about 0.09% to about 0.16% phosphorus, and the remainder is refractory metal.
In one aspect of the process the chemically-bound oxygen is present in the form of copper oxide. In another aspect of the process the green compact is exposed to moisture during sintering. The resulting sintered composite is a net shape or near net shape part that has a density that is at least 99% of theoretical, a porosity less than about 1%, and improved thermal conductivity compared to a similar copper composite that contains the same amount of sintering aid but no phosphorus and with the remainder being refractory metal.